Sen. Scott Brown says he has been stymied this year in his effort to build a record of bipartisan legislative successes — a problem Republicans blame on Democratic efforts to sink his 2012 re-election bid.

Democrats do not deny they have blocked his legislation but contend the Massachusetts Republican has faced trouble because his bills, while bipartisan in concept, have contained partisan elements that made them unacceptable to the majority Conference.

Republicans dismiss that explanation as semantics, arguing the Democrats' aim is to prevent Brown from having a record to run on in his overwhelmingly liberal state and in a race that presents the majority with perhaps its brightest opportunity to flip a GOP seat.

"It's sad; I think this is one of the reasons people don't like what's happening in Washington," Brown said during a recent interview after an amendment he filed to the defense authorization bill was nearly scuttled by Democrats. "It's just sad. There's plenty of time until the election."

Senate Armed Services ranking member John McCain (R-Ariz.), who managed the defense measure on a bipartisan basis with Chairman Carl Levin (D-Mich.), said he saw firsthand how Democrats used parliamentary and procedural tools to prevent Brown from getting approval for his proposals. For example, McCain said Brown's amendment to increase financial benefits for National Guardsmen who get deployed overseas was originally accepted by Democrats as part of standard negotiations governing which amendments would be allowed to receive a vote.

However, the amendment later became subject to an anonymous hold, with Democrats requesting that a proposal by Democratic Senatorial Campaign Committee Chairwoman Patty Murray (Wash.) be voted on in its place, McCain said. He added that her amendment had previously been rejected by the Republicans as unacceptable. Brown's amendment was salvaged after McCain threatened to derail the entire defense authorization bill, and it was easily adopted.

But McCain suggested that the manager's package of additional amendments died because it included a second Brown proposal — this one involving military tuition assistance — for which Democrats did not want the Massachusetts Republican to be able to take credit.

McCain said he has "no doubt" political considerations motivated Democrats to stall Brown's amendments.

"I don't know how you could draw any other conclusion. I've never seen anything quite like it," he said.

Brown faces a potentially tough general election matchup against Harvard professor Elizabeth Warren, and Democrats are eager to reclaim the seat held by "liberal lion" Sen. Edward Kennedy. Brown won the seat in 2010 in a special election upset that followed Kennedy's August 2009 death.

Republicans claim that Democrats have held up or moved to obstruct additional Brown proposals that had bipartisan appeal similar to his Defense authorization offerings, including a measure to repeal a law requiring a 3 percent withholding of payments to government contractors, the Hire a Hero Act providing tax credits to companies that hire military veterans and animal welfare legislation.

The withholding bill was taken from President Barack Obama's jobs proposal, and Republicans used the measure to highlight areas where they agreed with the president. Similarly, versions of the veterans jobs bill have been proposed by Republicans and Democrats alike.

Precisely because of the bipartisan support for those proposals, Republicans said it was obvious Democrats were moving to prevent Brown from getting credit.

Brown's version of the withholding repeal failed by just two votes, with Democratic co-sponsors, Sens. Mark Begich (Alaska) and Mark Pryor (Ark.) voting against it. A House Republican alternative of that bill later passed, but it was not Brown's bill.

The Senate passed the version of the Hire A Hero Act, but it was authored by Sen. Jon Tester (D-Mont.), who also faces a tough re-election race next year. Brown was officially a co-sponsor of the bill.

Asked to comment on the GOP complaints in general, Murray said: "I have no idea." The DSCC chairwoman said her focus in the Massachusetts race has been on Warren and the excellent qualities she brings to the race against Brown.

A Senate Democratic leadership aide conceded that Brown's proposals have been subject to roadblocks set up by the majority. But this aide attributed the Massachusetts Republican's legislative heartburn to how he has crafted his bills. Brown, the aide said, included partisan components unacceptable to Democrats within the body of his otherwise feel-good legislation, forcing Democrats to turn to substitute proposals.

The Senate Democratic leadership aide suggested that Brown has given Senate Minority Leader Mitch McConnell (R-Ky.) too much influence in the writing of his ostensibly noncontroversial legislation. For example, the aide said, the offset paying for the repeal of the 3 percent tax on contractors in Brown's original bill was politically partisan and therefore insufficient.

"The reason Scott Brown lacks accomplishments in the Senate is because he has been taking bad advice from Sen. McConnell. He took the ... idea from President Obama but then attached a poison pill offset to it," this aide said.

Brown's measure would have paid for the withholding tax repeal by rescinding $30 billion from fiscal 2012 discretionary spending.

National Republican Senatorial Committee Chairman John Cornyn (Texas) said the GOP has been able to marginalize Democratic efforts to sideline Brown but that it has been an uphill battle at times.

"It's all politics, unfortunately," Cornyn said. "It doesn't make any sense to me."